San Francisco developers are proposing to build the nation's tallest towers outside of New York and Chicago -- a pair of slender high-rises 350 feet taller than the Transamerica Pyramid.

The plan presented Thursday to the city's Planning Department envisions a cluster of thin towers rising from 2 acres at the northwest corner of First and Mission streets. The cluster would include two 1,200-foot towers, two 900-foot structures and a 600-foot companion.

Threaded between them would be an open plaza, covered passageways and a three-story building that is not part of the project.

By comparison, the Transamerica Pyramid is 853 feet high and the Bank of America building is 779 feet. The only U.S. buildings taller than those proposed Thursday are Sears Tower in Chicago and New York's Empire State Building, which are 1,451 feet and 1,250 feet respectively.

Though unprecedented for San Francisco, the proposal is in line with what city officials have been saying for months -- that extremely tall towers will be allowed on a handful of sites south of Market Street. But details of the project are likely to change during the city's review process, which could take at least two years.

Indeed, one member of the development team on Thursday described the "environmental evaluation application" presented to the city as "a placeholder."

"It is highly conceptual at this point," said Mark Solit, the lead developer. "Conceptual in terms of our discussion with the city, and conceptual in terms of the architects' vision of what they think might be appropriate."

The site is across from the Transbay Terminal, itself the focus of a skyscraper design competition seeking what the guidelines describe as "an iconic presence that will redefine the city's skyline." As many as a half-dozen teams are rumored to be putting together bids.

City planners earlier this year suggested raising building heights around the terminal as a way to attract projects that in turn would generate tax revenue. That money could then be used for the terminal and related transit projects such as an extension of commuter rail lines from the Peninsula.

The lead architect for the proposed cluster of towers is Renzo Piano of Italy, who also is doing the new home of the California Academy of Sciences in Golden Gate Park.

Piano has likened the design approach to bamboo shoots rising from the ground, with different pieces stopping at different heights. The two tallest would be on First Street -- rising 1,200 feet on either side of the Jessie Street alleyway.

The height would be accented even more by the narrow dimensions of each tower. On the top 300 feet of the tallest towers, the floors would measure just 8,000 square feet -- less than half the size of the upper floors one block away at Fremont Center. That 600-foot-high office tower is currently the tallest high-rise south of Market Street.

The development site is now parking lots and four six-story buildings built in the decade after the 1906 earthquake.

According to the application, the new buildings would contain 600 residential units, 470 hotel rooms, 520,000 square feet of office space and a small amount of ground-floor retail space. However, Solit said, the final mix would evolve along with the project.

Any project of this scale will require detailed studies of how the buildings will affect the wind and block sunlight, as well as engineering studies to confirm that such tall, narrow towers can withstand a major earthquake.

During the past week, Solit and other members of the development team have shown the project to Supervisors Chris Daly and Aaron Peskin and members of Mayor Gavin Newsom's administration. Full architectural details are not expected before summer.

"If we're going to do these kinds of heights, this is the place," said Daly, who also is a member of the Transbay Joint Powers Authority, which will oversee construction of a new terminal. "I like how the project works on the ground -- it's very porous and attractive to people on the street."

Daly suggested the most controversial aspect of the proposal could be the twin 1,200-foot towers.

"Every American is going to look at them and think of 9/11," he said.

Whatever form the project eventually takes, it shows that decision-makers no longer see dramatic building heights as something to avoid.

This wasn't the case in the decades after the Transamerica Pyramid began construction in 1970; that concrete spike at the foot of Columbus Avenue crystallized opposition to the transformation of San Francisco's skyline. An urban design plan the next year capped heights at 700 feet, and a 1986 update sliced off another 100 feet.

In recent years, though, the city has allowed residential towers in areas that before were kept low -- such as the towers now rising north of the Bay Bridge. Three are under construction, and two will top 600 feet.

San Francisco isn't the only city where the sky is now the limit.

Piano has 1,000-foot buildings in the works for the centers of both London and Boston -- two cities once as tower-wary as San Francisco. In Paris, a 984-foot tower proposal was announced last month for a site 3 miles west of the Eiffel Tower. The architect is Thom Mayne of Santa Monica, who designed the soon-to-open federal complex at Seventh and Mission streets in San Francisco.